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    Debugging Mozilla with lldb


    Mozilla-specific lldb settings

    There's an .lldbinit file in the Mozilla source tree, which applies recommended settings and includes a few type summaries and Mozilla-specific debugging commands via the lldbutils module (see python/lldbutils/README.txt). For information about available features see the links above and Using LLDB to debug Gecko blog post.

    The in-tree .lldbinit should be loaded automatically in most cases when runnning lldb from the command line (e.g. using mach), but not when using XCode. See Debugging on Mac OS X for information on setting up XCode.

    LLDB warning: Xcode 5 only comes with lldb (gdb is gone). The introduction and use of UNIFIED_SOURCES in the source starting around November 2013 has broken the default LLDB configuration so that it will not manage to resolve breakpoints in files that are build using UNIFIED_SOURCES (the breakpoints will be listed as "pending", and lldb will not stop at them). To fix this add the following to your $HOME/.lldbinit file:

    # Mozilla's use of UNIFIED_SOURCES to include multiple source files into a
    # single compiled file breaks lldb breakpoint setting. This works around that.
    # See for more.
    settings set target.inline-breakpoint-strategy always

    Restart Xcode/lldb and restart your debugging session. If that still doesn't fix things then try closing Xcode/lldb, doing a clobber build, reopening Xcode/lldb, and restarting your debugging session.

    Starting a debugging session

    Attaching to an existing process

    You can attach to Firefox with following command:

    (lldb) process attach --name firefox

    Some versions of lldb causes crashes after attaching to Firefox.

    Running a new process

    To start Firefox under the debugger, run lldb followed by "--", followed by the command line you'd like to run, like this:

    $ lldb -- obj-ff-dbg/dist/ -no-remote -profile /path/to/profile

    Then set breakpoints you need and start the process:

    (lldb) breakpoint set --name nsInProcessTabChildGlobal::InitTabChildGlobal
    Breakpoint created: 1: name = 'nsInProcessTabChildGlobal::InitTabChildGlobal', locations = 0 (pending)
    WARNING:  Unable to resolve breakpoint to any actual locations.
    (lldb) r
    Process 7602 launched: '/.../obj-ff-opt/dist/' (x86_64)
    1 location  added to breakpoint 1

    Document Tags and Contributors

    Contributors to this page: Jonathan_Watt, Nickolay, jrmuizel
    Last updated by: Nickolay,